Generally, a vehicle can be provided with a passenger protection device such as side airbags to protect passengers from a side collision between the vehicle side surface and an obstacle. The passenger protection device is actuated when the side collision has been detected. For example, as disclosed in JP-2-249740A, pressure sensors are provided to detect pressure variations (due to collision load) in closed spaces (air tanks) in side doors of the vehicle, and the side airbags are deployed based on detection signals of the pressure sensors.
A collision detection system according to a related art will be described with reference to FIG. 7. As shown in FIG. 7, doors 102-105 of a vehicle are respectively provided with side airbags 112-115 as the passenger protection device for the side collision. Pressure sensors 108a, 108b, 108c and 108d are respectively arranged in spaces defined in the doors 102-105 to detect inner pressures (which will vary due to side collision of vehicle) of the doors 102-105. Moreover, the pressure sensors 108a-108d are electrically connected with a collision detection ECU 109, respectively through wires 110a, 110b, 110c and 110d. 
That is, in this case, the number of the pressure sensors is the same with that of the doors of the vehicle. Thus, it is also necessary to provide the pressure sensors with the wires which have the same number with the doors. Therefore, the cost of the collision detection system is increased.